This is Online Notes on Chapter=5 ( Periodic Classificatoin of Elements) Part 2 for preparation of CBSE BOARD Examination, NTSE etc.=> Modern Periodic Table:
• Atomic number of an element is a more fundamental property than its
atomic mass.
• According to the Modern Periodic law : The properties of elements are a
periodic function of their atomic number.
• All the anomalies of Mendeleev’s classification disappear.
=> Merits of Mendeleev’s Periodic Table:
(i) Some gaps were left for the undiscovered elements like gallium (Ga),
Scandium (Sc) and Germanium (Ge).
(ii) Predict properties of elements on the basis of their positions in the
periodic table.
(iii) Accommodate noble gases when they were discovered without
disturbing the original arrangement.
=> Limitations of Mendeleev’s Classification:
(i) Position of isotopes could not be explained.
(ii) No fixed position for hydrogen.
(iii) Wrong order of atomic masses of some elements could not be explained.
Explanation of the Anomalies :
(i) Explanation for the position of isotopes (Same atomic number put at one
place in the same group).
(ii) Cobalt with atomic number 27 came first and nickel (28) should come
later.
(iii) Unlike atomic masses, atomic number is always a whole number, so
there is no element between hydrogen and helium.
• Atomic Number : It is denoted by Z and equal to the number of protons
in the nucleus of an atom.
• Modern Periodic table has 18 vertical columns known as ‘groups’ and 7
horizontal rows known as ‘periods’.
• Elements with same number of valence electrons are placed in the same
group. For example,
Li : 2, 1 Na : 2, 8, 1 K : 2, 8, 8, 1
Outermost or valence shell in all the three contains 1 electron. These elements
have been placed in the same group.
• Number of shells increases as we go down the group.
• Elements with same number of occupied shells are placed in same period.
For example, Li (2, 1); Be (2, 2); B (2, 3), C (2, 4), N(2, 5). These
elements have same number of shells (two).
• Each period marks a new electronic shell getting filled.
• Number of elements placed in a particular period depends upon the fact
that how electrons are filled into various shell.
• Maximum number of electrons that can be filled in a shell is given by 2n2
where n is shell number.
E.g., K shell n = 1 or 2n2 = 2(1)2 = 2 First period has 2 elements.
L shell n = 2 or 2n2 = 2(2)2 = 8 Second period has 8 elements.
• Position of an element in the periodic table tells us its chemical reactivity.
• Valence electron determine the kind and number of bonds formed by the
element.
=> Trends in the Modern Periodic Table:
Valency : No. of valence electrons present in the outermost shell of its atom.
On moving from left to right in each period, the valency of elements increases
from 1 to 4 and then decreases to 0.
Metallic Character:
• Metallic character means the tendency of an atom to lose electron.
• Metals occupy the left hand side of the periodic table.
• On moving left to right in a period, the metallic character of an element
decreases because the effective nuclear charge increases. It means
tendency to lose electron decreases.
• Metals are electropositive as they tend to lose electrons while forming
bonds.
• Metallic character increases as we go down a group as the effective
nuclear charge is decreasing.
Non-metallic Character
• Non-metals are electronegative as they tend to form bonds by gaining
electrons.
• Non-metals occupies the right side of the periodic table.
• Non-metallic character increases across a period because due to increase
in effective nuclear charge that means tendency to gain electron increase.
• Non-metallic character decreases as we move down a group due to
decrease in effective nuclear charge experienced by the valence electron
thus the tendency to gain electron decreases.
• In the middle of periodic table we have semi-metals or metalloid because
they exhibit some properties of metals and non-metals.
• Oxides of metals are basic in nature while oxides of non-metals are acidic
in nature.
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